In many instances such as retail or trade fairs it is desirable to present articles, e.g. fresh food, in an attractive way. With regard to illumination, usually this means that the colors of the articles should be enhanced, in other words that the articles should produce more saturation of color. Today usually compact high intensity discharge lamps, such as ultra high pressure sodium lamps (HID lamps) or special fluorescent lamps are used for this purpose. In the case of an ultra high pressure sodium lamp an additional filter is often used to obtain the required saturation, leading however to low system efficacy. Furthermore, since the emitted light is not white, a HID lamp must be placed near the merchandise to be illuminated, so that other objects are not illuminated unfavorably. Another disadvantage of today's HID lamps is that they emit a lot of heat, which is undesirable e.g. in the illumination of food articles, since the shelf life of the articles is reduced. Additionally the ultra high pressure sodium lamps have a short life (approximately 6000 hours) and are not stable in color over this life span.
Drawbacks of the fluorescent lamps include their linear size and length, limiting the application possibilities.
A light-emitting diode (LED) based solution can in principle be used to overcome the above disadvantages. By combining LEDs having different spectral outputs in the desired proportion, e.g. blue, green, amber and red, a total spectral output giving saturation of certain colors can be obtained. However, drawbacks of this solution include low efficiency and complexity of the system, as the use of different colors of LEDs leads to complex binning issues. In general lighting applications, some disadvantages of systems with LEDs of different colors can be overcome by using only blue LEDs and conversion of part of the blue light by a phosphor (wavelength converting material) to obtain white light output. However, White LEDs based on blue light converting phosphors generally exhibit a broad emission spectrum, and thus high saturation of colors cannot be achieved.
Thus, there is a need in the art for an improved illumination device by which high saturation of colors can be achieved, which is efficient and which exhibits acceptable color stability.